This paper discusses the interpersonal dimension of self-tracking technologies from the standpoint of Dewey’s pragmatist ethics. Users of self-tracking routinely exchange data with others, interact through social features embedded in their tools, and form communities focused on the sharing and discussion of data. I employ Dewey’s notion of transaction to discuss how self-quantification impacts users’ perception of others and how it mediates interpersonal relations. In Dewey’s ethics engagement with others is a fundamental part of moral life and individual flourishing can (...) only be achieved in social contexts and through the extension of care towards other people. The ethical ideals and epistemic concepts present in pragmatism allow me to argue that despite claims that self-tracking technologies lead to the formation of communities, pro-social instincts exist prior to the adoption of the devices and self-tracking actually inhibits the formation of mutually uplifting social bonds. In my discussion, I focus on the degree of epistemic distance that makes genuine engagement with others difficult, limits to transactional agency that prohibits users from deciding how they are perceived by others, and the reductive and instrumental nature of interactions embedded in the social features of self-tracking technologies. Moreover, I also analyse some political aspects of community-formation in the context of self-quantification and criticise the ownership structures of data and infrastructure that limit communities’ political agency. (shrink)